We are studying biochemical events characteristic of malignant tumor cells, which must be highly motile while invading tissue and metastasizing to distant sites. We have found that a number of metastatic cell lines produce and respond to autocrine motility factors. A partially purified material from the conditioned media of a human melanoma cell line was found to be a protein with an Mr of about about 55 KD. The material induces a strong chemotactic response in the producer cells and appears to exert its action by perturbing membrane phospholipid metabolism of the cell. Further studies on the mechanism of action of the motility factor indicate that Gi proteins are involved in the amplification of the motility signal while the Gs proteins may be involved in modulation of the signal. Other investigations on factors affecting regulation of motility indicated that cathepsin B in the cell membrane may play a role in both the adherence and migration of melanoma cells. In studies on the occurrence of motility factors in tumor cells, we have found that a variety of transformed cells including both metastatic and nonmetastatic produce and respond to motility factors. In addition, these cells as well as some revertant lines migrate to the melanoma factor. We have also developed further purification procedures in producing melanoma motility factor. These will assist us in the preparation of sufficient material to clone the gene for the factor and subsequent studies on its role in metastasis.